1. Field of the Invention
The present invention relates to a folded antenna wherein the physical length of along the axial direction of the antenna can be made short, adjustment of multiple resonant frequencies is easy, and transmitting and receiving at these multiple desired frequencies can be carried out with high gain. Furthermore, the present invention relates to an antenna device using the folded antenna, capable of standby-receiving at multiple desired frequencies and obtaining high antenna gain when in an extended state. Moreover, the present invention relates to a radio using the antenna device which is suitable for use in a dual band mobile telephone or the like.
2. Description of the Related Art
FIG. 19 shows an antenna device previously proposed by the present inventors in Japanese Patent Application No. 160016/1996. As shown in FIG. 19, this antenna device comprises a folded antenna 10, a whip antenna element 12 and a helical antenna element 14. The folded antenna 10 comprises a wire-like or belt-like conductor, which is provided along a direction from the base to the tip side, folded at the tip side parallel to the direction from the base to the tip side, and then folded again in parallel at the base side, ending with the tip facing the tip side. Then, the conductor is arranged in the shape of a cylinder having an axis in the direction from the base to the tip side. Furthermore, the helical antenna element 14 is provided on the tip of the whip antenna element 12 along the same axis and in a single body therewith, and this single body is freely extendable from and storable in the cylindrical folded antenna 10 along the axial direction thereof. Moreover, in the extended state, the base portion of the whip antenna element 12 is capacitance-coupled with the tip portion of the cylindrical folded antenna 10.
Then, the effective length of the folded antenna 10 from base to tip is set to a quarter of the wavelength of a first frequency f1. Here, as a result of floating capacitance between wires which have been folded parallel to each other, the folded antenna 10 acts as an antenna longer than its actual physical length. Furthermore, the effective length from the base to the first fold is set to a quarter of the wavelength of a second frequency f2, and the effective length from the base to the tip is set to three quarters of the wavelength of the second frequency f2. The second frequency f2 is higher than the first frequency f1, and as a result the floating capacitance between the parallel wires increases, thereby making the effective length even longer than the physical length. Therefore, the folded antenna 10, for which the first frequency f1 is resonant, can resonate the second frequency f2, which is lower than three times the first frequency f1. Then, as shown in FIG. 20, by setting the floating capacitance between parallel wires to an appropriate value, it is possible to set the second frequency f2 to approximately twice the first frequency f1.
Furthermore, the effective length from the base of the whip antenna element 12 to the tip of the helical antenna element 14 is set to half the wavelength of the first frequency f1, and the effective length from the base of the whip antenna element 12 to the tip thereof is set to half the wavelength of the second frequency f2.
As shown in FIG. 19, in this constitution, when the whip antenna element 12 and the helical antenna element 14 are extended from the folded antenna 10, at the first frequency f1, maximum voltage occurs at the tip of the folded antenna 10, and the base portion of the whip antenna element 12 and the tip of the folded antenna 10 become electrically connected at high frequency by a coupling capacitance C1, making it possible to transmit and receive at the first frequency f1. Furthermore, at the second frequency f2, maximum voltage occurs at the first fold point of the folded antenna 10, and the base portion of the whip antenna element 12 and the first fold point of the folded antenna 10 become electrically connected at high frequency by a coupling capacitance C2, making it possible to transmit and receive at the second frequency f2. At the second frequency f2, the helical antenna element 14 acts as a choke coil, not as an antenna. In the stored state, it is possible to transmit and receive at the first frequency f1 and the second frequency f2 using only the folded antenna 10.
When the first frequency f1 is set within a 900 MHz band and the second frequency f2 is set within a 180 MHz band, it is possible to transmit and receive at dual-band, such as GM/DCS or PDC/PHS, using a single antenna device. In this way, the previously proposed technology can also accommodate dual-band transmission and reception, and standby-reception in the stored state, and in addition, can obtain high gain antenna characteristics in the extended state.
However, in the previously proposed technology, the first frequency f1 and the second frequency f2 are both resonated by the folded antenna 10, comprising one conductor which is folded as appropriate. Consequently, when changing the physical length to the tip of the folded antenna 10, or the distance between the parallel wires or the length of the parallel portion, or the length to the first folding portion of the folded antenna 10, or the like, in order to adjust one of the resonant frequencies, there is an effect on the other resonant frequency, making it difficult to adjust the first frequency f1 and the second frequency f2 to desired frequencies. Furthermore, although the input/output impedances at the base of the folded antenna 10 can be adjusted by adjusting the coupling capacitances C1 and C2, it is difficult to adjust them individually, and consequently difficult to adjust them both to an optimum level. Moreover, since the length from the base to the first fold point is specified to a quarter of the high frequency (namely, the second frequency f2), the folded antenna 10 cannot be made shorter in the axial direction.